The park covers 172,971 acres and includes Elliott Key, the park's largest island and first of the true Florida Keys, formed from fossilized coral reef. The shoreline swamps of the mainland and island margins provide a nursery for larval and juvenile fish, molluscs and crustaceans. Mark C. Honeywell's guesthouse on Boca Chita Key was the area's most elaborate private retreat, featuring a mock lighthouse.